Over voltage on supply line, exceeding max voltage on an I/O, negative voltage on I/O (-0.3V is typical limit), reverse voltage on supply lines.
Over-Temperature is also a possibility (either due to environment or internal power dissipation or both). It does a pretty good job of limiting excessive current on outputs, but if total power dissipation is exceeded, that could cause failure (especially at high ambient temps). Also, floating inputs can cause it to exceed power ratings.
Not sure, but running it at too high of a frequency might axe it as well.
BUT, most of this is conjecture, because they have rarely failed, and only because of "bench accidents" during R&D
